C/SiC materials (ceramic materials which are reinforced with carbon fibers and have a matrix comprising SiC) for the shaped body of the invention include, inter alia, the materials described in the following documents: DE-A 198 56 721, DE-A 197 11 829 and DE-A 197 10 105.
In general, C/SiC is produced by firstly producing an open-pored carbon-containing intermediate body comprising reinforcing fibers and a carbon-containing matrix, generally a body made of a carbon material reinforced with carbon fibers (known as CFC or C/C). This is shaped to bring it close to its final shape and then infiltrated with a silicon melt at temperatures of about 1600° C. under reduced pressure or under inert gas, which results in at least part of the carbon of the matrix and/or of the fibers being converted into SiC. This forms a dense, strong and very hard material containing fibers, generally carbon fibers, with a matrix consisting predominantly of SiC, Si and C. Owing to the liquid silicization process, the material has to be cooled from at least about 1600° C. to room temperature after it has been produced. Thermal shrinkage takes place within this temperature range and this can be significantly different for each of the materials present depending on their coefficient of thermal expansion.
In the production of high-performance brake disks or clutch disks made of C/SiC, it is customary to provide the core body made of a base material with a covering layer having particular frictional properties. Such a brake disk having a wear-resistant covering layer (friction layer) and methods of producing it are known from, inter alia, DE-A 44 38 455, but this does not discuss the differences between the material of the core body and the friction layer.
It is known that the frictional properties and their uniformity can be improved by increasing the SiC content of the friction layer and that the wear of the components can be reduced thereby. DE-A 198 05 868 and EP-A-0 818 636 describe generic brake disks having C/SiC friction layers which have an SiC content which is higher than that of the core body and can be up to almost 100%. The friction layers are produced by silicization of C/C intermediate bodies, with the higher SiC content of the friction layer being achieved by means of the degree of conversion in the silicization reaction of the C/C to form SiC; in the first case by means of a higher reactivity of the friction layer C/Cs and in the second case by a reduction in the silicon available in the core body.
C/SiC materials of construction produced by the liquid silicization method are also known from, for example,
DE-A 199 53 259. For particular applications, e.g. protective plates, the application of hard material layers having a high SiC content is desirable so as to improve the properties.
The typical coefficient of thermal expansion of these SiC-rich covering layers comprising SiC, SiSiC or C/SiC is in the range from about 2.5 to 5×10−6 K−1. The typical coefficient of thermal expansion of the C/SiC core body which is lower in silicon carbide is, on the other hand, in the range from about 0.5 to 3×10−6 K−1.
During the cooling from the silicon melt temperature (about 1420° C.) to room temperature (about 20° C.) necessary in the liquid silicization production process, the different coefficients of thermal expansion lead to thermally induced stresses which result in a pronounced pattern of cracks in the layer which are visible to the eye.
This pattern of cracks is usually completely random and is characterized by a low degree of order.
The load states which are usual for brake and clutch disks are characterized by ongoing heating and cooling processes, with continuing cyclic thermal stress being general during use of the components for the purpose for which they are designed. The high thermally induced stress peaks are dissipated in the material by the formation of further cracks or by growth of existing cracks.
In the case of metallic materials, the accumulation and growth of cracks give information on the state of wear or damage to the material, so that the cracks can be employed as an early indicator of possible mechanical failure of a component.
A similar type of behavior is displayed by fiber-reinforced ceramic composites, with the formation of new cracks occurring to a much greater extent than the growth of existing cracks.
A disadvantage of the pattern of cracks known from the prior art in SiC-rich covering layers is that the cracks arising from the production process itself cannot be unambiguously distinguished from cracks which are newly formed as a result of cyclic thermal stress. The visible cracks can therefore not be employed reliably as an early indicator of wear of the material.
In the case of C/SiC construction materials, a disordered pattern of cracks is undesirable for reasons of aesthetics, in the case of protective plates particularly because of the required function, since it gives an impression of lower product quality. It is therefore desirable to avoid the formation of cracks completely or at least to limit them to defined, restricted regions.